First, to explain some backstory of the Palantiri.
The Palantíri were made by the Ñoldor in Eldamar, possibly by Fëanor himself in Aman during the Elder Days in the Time of the Trees, and then given by the Elves to the Númenóreans, who kept them as heirlooms until the Fall of Númenor during the Second Age. Then seven of these stones were rescued and brought to Middle-earth by Elendil and his sons.
The Dúnedain placed the stones across large distances in order to communicate with one another.
The stones were housed at the following locations: Annúminas, Amon Sûl (Weathertop) and Elostirion (Tower Hills) in the north, and Osgiliath, Orthanc (Isengard), Minas Ithil, and Minas Anor in the south. The Master Stone resides in Tol Eressëa, in the Tower of Avallonë.
Four of the stones are known to have been lost. The chief stone of the north at Amon Sul, along with the stone from Annúminas was lost with Arvedui in the cold northern seas. The chief stone of the south in Osgiliath was lost during the Kin-strife. The stone of Minas Ithil was captured by Sauron and was very likely destroyed during the destruction of Barad-dûr.
The stone of Elostirion was taken back to the Undying Lands (Valinor) on the Ringbearers' ship. Only two stones remain in Middle-earth; the stones of Minas Anor and Orthanc, yet the stone of Minas Anor was marred, showing all but the most strong-willed the sorrows and madness of Denethor II.
Back in the day, the Palantiri were most readily available to heirs or kings, as well as those appointed to guard them. The stones were able to sense their user and allow use based on their position; hence Denethor could utilize the stone of the White City easily, whereas Saruman struggled with its use, and was eventually overcome by Sauron. Palantíri, wikia
So to the question proper; The Palantir that Saruman had in Orthanc was in due course delivered to Aragorn as was stone Denethor had in Minas Tirith (Minas Anor). That leaves the reigning King in middle earth with two of the three surviving Palantiri and the third taken across the sea to the Undying lands. Why was the Palantir of Elostirion taken back to the Undying Lands? Would this mean that Aragorn had ongoing communications with Valinor?